Since the advent of stereophonic sound in the first half of the 20th century, audio engineers have been attempting to create illusions of multidirectional audible perception. What originally began as two loud speakers playing two different audio channels has evolved into sophisticated and complicated systems and algorithms that try to create an environment for a user that simulates real life audio through a speaker system. The goal is to provide a user with an immersive experience where they are enveloped by life-like sound.
A typical stereo sound reproduction system is generally made up of two loudspeakers. One loudspeaker is on the left and one is on the right. Each loudspeaker unit is usually made up of an electromagnetic driver assembly commonly called a speaker that generates sound and an acoustic enclosure which serves as a housing for the entire unit. The electromagnetic driver assembly has a moving part, often referred to as a voice coil, which is attached directly to a voice diaphragm. The moving coil is responsive to an electrical signal by moving forward and rearward along its central axis. The electrical signal is translated into motion of the diaphragm. Forward motion of the diaphragm carries with it a volume of air which produces audible sound which is directed towards the listener. The rearward motion of the voice diaphragm and coil assembly also produces sound but is out of phase relative to the sound generated during the forward motion. The sound created during the rearward motion is contained within the enclosure. When appropriately designed, only the sound during forward motion is heard.
Another type of a loudspeaker is an exciter which operates by bending waves. The bending wave application requires an electromagnetic driver but without the typical voice diaphragm found on conventional speakers. The driver's moving coil mechanism is instead mounted on to the back of a board. The board is similar to the loudspeaker diaphragm in that an audio signal causes the voice coil mechanism to move forward and rearward. The front of the board pushes the air towards the listener. The driver, or exciter, can advantageously be mounted inside the wall so it doesn't need an enclosure box so that it can be hidden from plain sight and can save space.
In an attempt to create a sense of spatial presence, loudspeaker designers have developed passive acoustic lenses to divert sound directionality so that a listener can experience omnidirectional sound instead of hearing directly from the speaker. Some acoustic lenses are shaped like a cone where the tip is oriented directly toward the center of a loudspeaker diaphragm at a very close proximity. This setup usually has the loudspeakers facing upwards so the reflected sound to disperse radially in horizontal orientation. The method has been created by Zenith in the Zenith Circle of Sound and by various other companies with designs of their own.
Designing an acoustic enclosure for conventional speakers has proven to be very challenging and typically adds significant amount of volumetric size and complexity. An enclosure is usually needed to isolate the rear facing portion of the electromagnetic driver assembly for sound cancelation purposes. The sound cancelations reduce the loudspeaker's overall efficiency and sound quality. Both conventional loudspeaker type and the bending wave type of technologies share the same drawbacks. As the voice diaphragm or the vibrating plane increase in surface area, the upper frequency of the audio spectrum is greatly reduced. On the other hand, as their surface areas decrease, the lower frequency of the audio spectrum is also greatly reduced. As a result, drivers such as tweeters and or subwoofers are added to the system to extend the audible frequency coverage. However, the added number of drivers also adversely affects the sound quality if the frequency distribution between drivers is not completely seamless.
Both technologies mentioned above when used in a stereo arrangement can deliver quality sound to the listener. However, the sound can only be projected within the left and right regions. The projection results in a confined sound and lacks the ability of presenting the near and far regions relative to the listener's location. Both technologies present a sound image analogous to a two-dimensional photograph of a three-dimensional object.
Passive acoustic lenses of various shapes have also been implemented in sound projections. While the results may present some improvements in sound quality, the application is best suited for optimizing sound dispersion to focus the sound where it is needed. Differentiating sound locations from left, right, near, and far regions is not easily achievable and cannot be achieved with conventional loudspeakers. For example, to get more out of the antiquated loudspeaker technology, the design type that adheres to the concept of projecting sound towards the listener whether directly or indirectly, the BACCH 3D sound system was developed by Professor Edgar Choueiri at Princeton University to address the technology's spatial limitation. This system is made up of filters and is associated with complex computer algorithm and/or apparatus to track the listener's position as a way to tailor the sound through existing stereo loudspeakers. The system is implemented between the media playback device and the loudspeakers to modify certain sound properties of the original recording for crosstalk cancellation. For the intended 3D sound to be fully experienced, the two speakers must also be at a prescribed distance apart and the listener's location or “sweet spot” relative to the speakers is precisely determined using specialized apparatus needed to maintain the 3D sound presentation. This special method of crosstalk cancellation at the signal chain in an audio system along with the necessary apparatus that make adjustments to compensate for listener positioning in order to achieve the intended 3D sound can be a very expensive solution especially for an average consumer.
Therefore, there is a need for a system that can produce more realistic sound imaging. It is further desirable to be able to produce a system that is capable of transmitting a three-dimensional sound presentation in a simple and affordable manner.